According to the gradual increase in demand for a rubber composition in various manufacturing fields for tires, impact resistant polystyrene, the sole of shoes, golf balls, etc., the value of conjugated diene-based polymer which is a synthetic rubber, specifically, a butadiene-based polymer is increasing as an alternative material to natural rubber of which the produced amount is insufficient.
Meanwhile, in a conjugated diene-based polymer, linearity and the degree of branching greatly affect the physical properties of the polymer. In particular, with a decrease in linearity or an increase in the degree of branching, the dissolution rate and viscosity properties of the polymer increase, and as a result, the processability of the polymer is improved. However, if the degree of branching of the polymer is excessively large, molecular weight distribution is broadened, and the mechanical properties of the polymer, which influence the abrasion resistance, cracking resistance or repellency of a rubber composition may rather be deteriorated. In addition, the linearity and the degree of branching of the conjugated diene-based polymer, specifically, the butadiene-based polymer are highly dependent on the amount of cis-1,4 bonds contained in the polymer. If the amount of cis-1,4 bonds contained in the conjugated diene-based polymer increases, the linearity may increase. As a result, the polymer has good mechanical properties, thereby increasing the abrasion resistance, cracking resistance and repellency of a rubber composition.
Accordingly, various methods for preparing a conjugated diene-based polymer are being studied and developed to increase the amount of cis-1,4 bonds in a conjugated diene-based polymer and increase linearity and such that the conjugated diene-based polymer will have appropriate processability at the same time.
Particularly, a method of preparing a butadiene-based polymer using a compound of a rare earth metal such as neodymium and an alkylating agent in group I to group III, particularly, a polymerization catalyst of a composite metal composed of methyl aluminoxane has been developed. However, a polymer obtainable by the method has an insufficiently high amount of cis-1,4 bond, and an insufficiently small amount of vinyl, such that the improving effect of physical properties is still insufficient.
As an another method, a method of preparing a butadiene-based polymer having a high amount of cis-1,4 bonds using a polymerization catalyst including a rare earth metal compound, an alkylating agent in group I to group III, and an ionic compound composed of non-coordinating anions and cations, has been developed. In the method, Nd(OCOCCl3)3 is used as the rare earth metal compound, but the polymerization activity of the metal compound is low, and the amount of vinyl bonds of the butadiene polymer is large, such that a rubber composition including the butadiene-based polymer prepared by the method attained an insufficient improvement of physical properties when compared to a rubber composition including the conventional butadiene-based polymer. In addition, the butadiene-based polymer prepared by the method has a large amount of vinyl bonds and wide molecular weight distribution.
As another method, a method of preparing a butadiene-based polymer having a large amount of cis-1,4 bonds using a polymerization catalyst composed of a rare earth metal salt composed of a halogen atom-containing component and aluminoxane, has been developed. However, a specific catalyst such as neodymium bis(trichloroacetate) (versatic acid), etc. is used, such that the polymerization activity of a neodymium salt is low, and industrial applicability is low.
Therefore, the development of a method of preparing a conjugated diene-based polymer which has high linearity and is capable of showing excellent processability, is required.